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MgO Layer Thickness Dependence of Structureand Magnetic Properties of L10-FePt/MgO/GaAsStructures
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Japanese Journal of Applied Physics 51 (2012) 02BM05
DOI: 10.1143/JJAP.51.02BM05

MgO Layer Thickness Dependence of Structure and Magnetic Propertiesof L10 -FePt/MgO/GaAs Structures
Rento Ohsugi1 , Makoto Kohda1;2 , Takeshi Seki3 , Akihiko Ohtsu1 ,Masaki Mizuguchi3 , Koki Takanashi3 , and Junsaku Nitta1
1Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
3
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2

Received October 3, 2011; accepted October 18, 2011; published online February 20, 2012
We investigated the MgO layer thickness dependences of the structure and magnetic properties of L10 -FePt/MgO/GaAs structures. To examine
how the crystallinity and growth morphology of the MgO layer affect the L10 -FePt layer, two kinds of preparation method were employed for MgO
deposition: electron beam (EB) evaporation and sputter deposition. The MgO layer deposited by EB evaporation included a large strain becauseof the cube-on-cube epitaxial relationship despite a large lattice mismatch between MgO and GaAs. For the MgO layer prepared by sputtering, on
the other hand, an amorphous MgO layer was initially grown on the GaAs substrate. Subsequently, a crystalline MgO layer was grown in the (001)
direction. In the case of the EB-deposited MgO, as the MgO layer thickness increased, the degree of chemical order of the L10 -FePt layer
increased from 55 to 81% owing to the improvement of the crystallinity of the MgO layer. The improvement of chemical order also led to theincrease in the remanent magnetization of L10 -FePt from 84 to 98%. # 2012 The Japan Society of Applied Physics

1. Introduction

A ferromagnetic metal (FM)/semiconductor (SC) heterostructure is one of the important building blocks for
operating spintronic devices at room temperature. Electricalspin injection into an SC through a high-quality heterojunction is a fundamental requirement for FM/SC heterostructures. Recently, nonlocal spin injection and detection havebeen achieved in FM/SC structures such as Fe/GaAs1) andCoFe/GaAs.2) In these devices, however, an easy magnetization axis of ferromagnetic metal electrodes was alignedin the in-plane direction owing to the demagnetizing ﬁeld.
To utilize the spin degree of freedom by taking advantage ofspin orbit interaction for an electrical spin manipulation3)
and a reduced threshold current in a vertical-cavity surfaceemitting laser,4) a perpendicularly magnetized FM/SC structure is indispensable. An L10 -FePt alloy is one of the promising materials for injecting perpendicularly oriented electronspins at zero magnetic ﬁeld because L10 -FePt shows large
perpendicular magnetic anisotropy.5,6) In addition to this,an L10 -FePt layer is directly grown on a MgO layer, whichmeans that MgO is utilized as a tunnel barrier for theeﬃcient spin injection into an SC. By taking advantageof this feature, an electrical spin injection from L10 -FePt
into GaAs has been demonstrated at room temperature.7)
However, the remanent magnetization of L10 -FePt is low,
resulting in a low spin injection eﬃciency at zero magneticﬁeld. In a previous study, we grew L10 -FePt/MgO/GaAsstructures with a high remanent magnetization with asputtered MgO tunnel barrier layer.8) However, the qualitiesof the MgO layer and the interface between MgO and GaAs
have not been fully understood. It is essential to conﬁrm andimprove the interface quality for an eﬃcient spin injection.
In this study, we have investigated the MgO thickness
dependences of the magnetic properties of the L10 -FePt
layer in L10 -FePt/MgO/GaAs structures. Two kinds ofpreparation technique were employed for MgO deposition:
electron beam (EB) deposition and sputter deposition. Thestructural diﬀerence has also been compared between
EB-deposited MgO and sputtered MgO.
E-mail address: makoto@material.tohoku.ac.jp

Table I. Thickness of the FePt and MgO layers (in nm) for sputtered MgOand EB-deposited MgO samples.

FePt thickness
Sputtered MgO samples

5EB-deposited MgO samples

20MgO thickness352352. Sample Preparation and Growth Condition

To obtain a clean surface of GaAs (001) substrates, the GaAs
(001) substrates were cleaned in HCl : H2 O ¼ 1 : 1 liquid
for 2 min to remove oxides and –OH on their surface. Aftercleaning, the GaAs substrates were immediately transferred
into the prechamber and annealed for 10 min in vacuum at a
substrate temperature (TS ) of 400  C. For the MgO growth,
we employed two diﬀerent methods: the magnetron sputtering and the EB evaporation. The TS value of the sputtered
MgO and EB-deposited MgO layers were 350 and 400  C,
respectively. After the MgO growth, samples were transferred into an ultrahigh-vacuum sputtering chamber without
breaking the vacuum condition. Then, Fe and Pt were cosputtered on the MgO layer with Fe0:43 Pt0:57 composition.
The TS value during the FePt growth were 350  C for
sputtered MgO samples and 400  C for EB-deposited MgOsamples.
Table I shows a summary of the thicknesses of FePt andMgO layers for all the samples. To monitor the ﬂatness andthe growth direction of each layer, we used the reﬂection
high energy electron diﬀraction (RHEED). Structural analyses of the L10 -ordered FePt/MgO/GaAs ﬁlm were performed by –2 X-ray diﬀraction (XRD) measurement.
XRD measurement was performed with Cu K radiation,and its step of 2 was ﬁxed at 0.04 . To observe themorphology of L10 -FePt/MgO/GaAs interfaces, we conducted the cross-sectional transmission electron microscopy(TEM). The magnetic properties of L10 -FePt layers weremeasured using the polar magneto-optical Kerr eﬀect

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R. Ohsugi et al.

Fig. 1. (Color online) (a) XRD patterns of EB-deposited MgO samples
with (1) 5, (2) 3, and (3) 2 nm MgO thickness and of sputtered MgO samples
with (4) 5 and (5) 3 nm MgO thickness.

(PMOKE) with application of the external magnetic ﬁeld
normal to the ﬁlm plane.
To calculate the degree of chemical order parameter S ofL10 -FePt, we used the following equation:sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

A001
A001
¼ S;
A002
A002 theory
where A001 and A002 are the integrated intensities of the 001
superlattice and 002 fundamental peaks in XRD measurement, respectively. ðA001 =A002 Þtheory is the theoretical ratioof A001 to A002 for the fully ordered alloy at Fe0:43 Pt0:57
composition, which is 0.38.
3. Results and Discussion

From the XRD patterns of FePt/MgO/GaAs structures
shown in Fig. 1, a clear MgO 002 peak appears withincreasing MgO layer thickness, indicating the (001)orientation of the MgO layer on the GaAs (001) substrate.
In addition to the FePt 002 fundamental peak, the FePt 001and FePt 003 superlattice peaks are observed for all thesamples. It is found that the L10 -ordered FePt (001) layer
was grown on (001) MgO/(001) GaAs by both EB
deposition and sputtering processes. RHEED patterns arealso shown in the insets in Fig. 2(b). These clear streak
patterns indicate the ﬂat surface of each layer. From theresults of XRD and RHEED measurements, the cubeon-cube epitaxial relationship of ½100GaAs k ½100MgO k½100FePt and ð001ÞGaAs k ð001ÞMgO k ð001ÞFePt is found inthe EB-deposited MgO sample.
To examine the morphology of interfaces in theL10 -FePt/MgO/GaAs structures, cross-sectional TEM wasperformed. Figures 2(a) and 2(b) show the TEM images ofL10 -FePt/MgO/GaAs structures with a sputtered MgO layer
and an EB-deposited MgO layer, respectively. As shown inFig. 2(a), a thin amorphous MgO layer is observed at theMgO/GaAs interface of the sputtered MgO samples, and acrystalline MgO (001) texture is subsequently grown after
the formation of the amorphous MgO layer. The in-plane
crystal orientation of the MgO layer grown on the amorphous layer is not conﬁned, unlike that of the MgO layer
epitaxially grown on GaAs. In the case of the EB-deposited
MgO sample [Figs. 2(b) and 2(c)], on the other hand, there is
no formation of an amorphous layer at the interface. TheFig. 2. (Color online) Cross-sectional TEM images of L10 -FePt/MgO/GaAs structure. (a) L10 -FePt (5 nm)/sputtered MgO (5 nm)/GaAs sample.
(b) L10 -FePt (20 nm)/EB-deposited MgO (3 nm)/GaAs sample. (c) MgOand GaAs interface in EB-deposited MgO sample.

Table II. Lattice constant and FWHM in sputtered MgO and EBdeposited MgO layers evaluated from the MgO 002 peaks obtained fromXRD measurement.

C-axis latticeconstant(A)

FWHM(deg)

Sputtered MgO (5 nm)

4.213

1.055

EB-deposited MgO (5 nm)

4.200

2.170

root-mean-square roughness values of FePt/MgO interfaces
are calculated to be 0.62 and 0.19 nm for the sputtered andEB-deposited MgO samples, respectively. The EB-deposited
MgO sample has a ﬂat FePt/MgO interface compared withthe sputtered MgO sample.
 andSince the lattice mismatch between MgO (4.213 A)
GaAs (5.68 A) is over 25%, the cube-on-cube epitaxial
relationship could lead to a large strain accumulation in theMgO layer, which gives rise to a large dispersion of crystal
orientation. To understand the strain accumulation in EBdeposited MgO layer and sputtered MgO layer, the fullwidths at half maximum (FWHMs) of MgO 002 peaks areevaluated.

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R. Ohsugi et al.

Fig. 3. (Color online) (a) FWHMs of FePt 002 peaks, (b) degree of chemical order parameters S of L10 -FePt, (c) c-axis lattice constants, (d) PMOKE(MH) loops of both sputtered (left side) and EB-deposited (right side) MgO samples, and (e) remanent magnetization Mr =Ms .

Table II shows the c-axis lattice constants of the MgOlayers and FWHMs of MgO 002 peaks. The c-axis lattice
constants for both MgO layers are almost identical to eachother, the values of which are consistent with the bulk lattice However, the FWHM of theconstant of MgO (4.213 A).
EB-deposited 5 nm MgO layer (2.170 ) is twice larger thanthat of the sputtered 5 nm MgO layer (1.055 ). Such a peakbroadening of MgO 002 indicates the large dispersion of
the c-axis orientation for the EB-deposited MgO layer. Thisdiﬀerence in dispersions of crystal orientation between
the EB-deposited and sputtered samples results from thediﬀerent growth morphologies at the MgO/GaAs interface.
In the case of the sputtered MgO layer, the formation of theamorphous MgO layer excludes the strain accumulation atthe MgO/GaAs interface, and the subsequent crystalline
MgO layer formation results in the narrow XRD peaks.
On the other hand, the EB-deposited MgO layer has large
strain accumulation due to the epitaxial growth without theformation of the amorphous MgO layer, leading to the large
FWHM.
To understand the eﬀect of the strain at the MgO/GaAsinterface on the magnetic properties of L10 -FePt, the MgOlayer thickness dependences of the FWHM of the FePt 001
peak are evaluated for EB-deposited MgO samples as well
as sputtered MgO samples. Figure 3(a) shows the FWHMsof FePt 001 peaks as a function of the MgO layer thickness.
With increasing MgO layer thickness, the FWHM of FePt
decreases in the case of the EB-deposited MgO samples.
This implies that the dispersion of crystal orientation of theMgO layer aﬀected the c-axis distribution of the FePt layer.
The thick MgO layer improves the crystallinity of the FePt
layer. On the other hand, there is no change in the FWHM ofthe sputtered MgO samples owing to the exclusion of thestrain accumulation by the amorphous MgO layer. The strain
due to the lattice mismatch aﬀects the magnetic properties ofL10 -FePt.6)
Figures 3(b) and 3(c) show the degree of chemical orderparameters S and c-axis lattice constants of the FePt layers,respectively. For the EB-deposited MgO samples, when theMgO layer thickness increases, S increases from 0.55 to0.81, whereas the c-axis lattice constant decreases. Since
the c-axis of the FePt is reduced when the FePt transforms
from the disorder (fcc) phase to the order (L10 ) phase,5) theincrease in S is in agreement with the decrease in the c-axis
lattice constant of FePt. One of the reasons for the promotionof L10 -ordering is the improvement of the crystallinity of
the FePt layer with the thick MgO layer. The c-axis lattice
constants of FePt in sputtered MgO samples are larger thanthat in EB-deposited MgO samples, because the absence ofstrain accumulation in the sputtered MgO layer leads to asmaller a-axis than in the case of the EB-deposited MgOlayer.
Figure 3(d) shows the PMOKE loops for FePt layers
prepared on the sputtered MgO and EB-deposited MgOlayers. All the ﬁlms show high remanent magnetizationin the perpendicular direction, indicating the existence ofperpendicular magnetic anisotropy. However, the ratioof remanent magnetization to saturation magnetization(Mr =Ms ) gradually changes depending on the MgO layer
thickness. Figure 3(e) shows Mr =Ms as a function of theMgO layer thickness of the sputtered MgO and EBdeposited MgO samples. For the EB-deposited MgOsamples, Mr =Ms increases from 84 to 98% with increasing
MgO layer thickness. The increase in Mr =Ms is consistent

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with the MgO layer thickness dependence of S. For thesputtered MgO samples, Mr =Ms does not show a marked
change with MgO layer thickness. This tendency isinterpreted as being due to the fact that the strain is notaccumulated in the MgO (001) layer due to the formation ofthe amorphous MgO layer on the GaAs surface.
4. Conclusions

We investigated the MgO layer thickness dependences of thestructure and magnetic properties of L10 -FePt on MgO/GaAs structures. The MgO layers prepared by EB evaporation epitaxially grew on the GaAs (100) substrate while thesputtered MgO layer initially formed an amorphous MgO onthe GaAs surface. The cube-on-cube epitaxial relationshipgave rise to a large strain in the EB-deposited MgO layer,
and the magnetic properties of L10 -FePt changed withincreasing MgO layer thickness. The improvement of S andMr =Ms was observed for the EB-deposited MgO samples
with a thick MgO layer.

of Education, Culture, Sports, Science and Technology, by
the New Energy and Industrial Technology Development
Organization, and by the Strategic International Cooperative
Program—Joint Research Type Japanese–German Joint
Research in Japan Science and Technology Agency. Theauthors thank Dr. Takamichi Miyazaki for taking TEM
images and helpful discussion.

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# 2012 The Japan Society of Applied Physics